This invention relates to radio receiving antennas. In particular, it relates to a system for reducing flutter in receiving radio transmissions.
Flutter is a noticeable variation in the audio signal developed by a radio receiver. Flutter results from the fact that the typical radio antenna receives signals that are reflected from various objects. The reflections may be in addition to a signal received over a direct path or, in the case of a receiver that is shielded by some object from a direct path to the transmitter, the entire communication may result from reflected signals. Flutter occurs when relative motion among a transmitter, the receiver and one or more reflecting objects causes variation in the net signal developed on the receiving antenna. That signal may be caused to be low by the superposition of signals that are 180.degree. out or nearly 180.degree. out of phase. If relative motion causes the signals to go in and out of phase at an audible rate, then the appearance and disappearance of the signal in the receiver will cause a fluttering of the audible signal. The flutter interferes with communication because it is caused by the existence of time intervals, normally randomly spaced, in which quieting is reduced or lost in FM receivers and in which the gain of the AGC in an AM receiver is caused to approach its maximum value. Either of these conditions causes an audible noise burst.
It should also be evident that the phenomenon of flutter is related to the frequency of the signal that is received, since a change in position of a quarter wavelength will cause a change from a maximum to a minimum of a received signal. A change of a half wavelength will cause a reversal of phase. For this reason flutter begins to become of concern at broadcast frequencies of the order of 150 MHz, with a free-space wavelength of 2 meters. It is more of a problem for communication at frequencies of the order of 900 MHz for which the wavelength is about 33 centimeters. Frequencies above 900 MHz have not been used extensively for mobile and portable communications systems, but will present increasing problems with flutter when they are used.
Mobile and portable communications systems almost invariably use omnidirectional antennas because receivers in such systems are often in positions where the only signal they can receive is reflected. This is in contrast to communication between fixed stations in which the combination of directivity and high gain associated with directional antennas minimizes or eliminates the problem of flutter. The term "omnidirectional" is here used to refer to an antenna that has no favored azimuthal angles although it may well be limited to elevation angles that exclude a considerable cone about the zenith. Because these antennas are omnidirectional in azimuth, they combine relatively low gain with the possibility of superposing a plurality of reflected signals. Thus they are subject to flutter.
It is an object of the present invention to reduce flutter in radio receivers.
It is a further object of the present invention to provide a means of reducing flutter in mobile radios and base-station radios.
It is a further object of the present invention to reduce flutter in radio systems operating at and above 150 MHz.
Other objects will become apparent in the course of a detailed description of the invention.